Piston Integrated Variable Mass Load

ABSTRACT

Embodiments relate to relate to marine vibrators that incorporate one or more piston plates that act on the surrounding water to produce acoustic energy. An example marine vibrator may comprise: a containment housing; a piston plate; a fixture coupled to the containment housing; a mechanical spring element coupled to the piston plate and the fixture; a driver disposed in the marine vibrator, wherein the driver is coupled to the piston plate and the fixture; and a container coupled to the piston plate, wherein the container is configured to hold a variable mass load; wherein the marine vibrator has a resonance frequency selectable based at least in part on the variable mass load.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/462,052, filed on Aug. 18, 2014, which claims priority toU.S. Provisional Application No. 61/904,886, filed on Nov. 15, 2013, andis a continuation-in-part of U.S. Nonprovisional application Ser. No.14/284,847, filed on May 22, 2014, which claims priority to U.S.Provisional Application No. 61/880,561, filed on Sep. 20, 2013, theentire disclosures of which are incorporated herein by reference.

BACKGROUND

Embodiments relate generally to piston-type marine vibrators for marinegeophysical surveys. More particularly, embodiments relate to theaddition of a variable mass load to the outer piston plate of apiston-type marine vibrator to compensate for air-spring effects.

Sound sources are generally devices that generate acoustic energy. Oneuse of sound sources is in marine seismic surveying in which the soundsources may be employed to generate acoustic energy that travelsdownwardly through water and into subsurface rock. After interactingwith the subsurface rock, for example, at boundaries between differentsubsurface layers, some of the acoustic energy may be reflected backtoward the water surface and detected by specialized sensors, in thewater, typically either on the water bottom or towed on one or morestreamers. The detected energy may be used to infer certain propertiesof the subsurface rock, such as structure, mineral composition and fluidcontent, thereby providing information useful in the recovery ofhydrocarbons.

Most of the sound sources employed today in marine seismic surveying areof the impulsive type, in which efforts are made to generate as muchenergy as possible during as short a time span as possible. The mostcommonly used of these impulsive-type sources are air guns thattypically utilize compressed air to generate a sound wave. Otherexamples of impulsive-type sources include explosives and weight-dropimpulse sources. Another type of sound source that can be used in marineseismic surveying includes marine vibrators, such as hydraulicallypowered sources, electro-mechanical vibrators, electrical marine seismicvibrators, and sources employing piezoelectric or magnetostrictivematerial. Marine vibrators typically generate vibrations through a rangeof frequencies in a pattern known as a “sweep” or “chirp.”

Prior sound sources for use in marine seismic surveying have typicallybeen designed for relatively high-frequency operation (e.g., above 10Hz). However, it is well known that as sound waves travel through waterand through subsurface geological structures, higher frequency soundwaves may be attenuated more rapidly than lower frequency sound waves,and consequently, lower frequency sound waves can be transmitted overlonger distances through water and geological structures than can higherfrequency sound waves. Thus, efforts have been undertaken to developsound sources that can operate at lower frequencies. Very low frequencysources (“VLFS”) have been developed that typically have at least oneresonance frequency of about 10 Hz or lower. VLFS's are typicallycharacterized by having a source size that is very small as compared toa wavelength of sound for the VLFS. The source size for a VLFS istypically much less than 1/10^(th) of a wavelength and more typically onthe order of 1/100^(th) of a wavelength. For example, a source with amaximum dimension of 3 meters operating at 5 Hz is 1/100^(th) of awavelength in size.

In order to achieve a given level of output in the water, a marinevibrator typically needs to undergo a change in volume. In order to workat depth while minimizing structural weight, the marine vibrator may bepressure balanced with external hydrostatic pressure. As the internalgas (e.g., air) in the marine vibrator increases in pressure, the bulkmodulus (or “stiffness”) of the internal gas also rises. Increasing thebulk modulus of the internal gas also increases the air-spring effectwithin the marine vibrator. As used herein, the term “air spring” isdefined as an enclosed volume of air that may absorb shock orfluctuations of load due to the ability of the enclosed volume of air toresist compression and decompression. Increasing the stiffness of theair in the enclosed volume increases the air-spring effect and thus theability of the enclosed volume of air to resist compression anddecompression. This increase in the air-spring effect of the internalgas tends to be a function of the operating depth of the source.Further, the stiffness of the acoustic components of the marine vibratorand the internal gas are the primary determining factors in the marinevibrator's resonance frequency. Accordingly, the resonance frequencygenerated by the marine vibrator may undesirably increase when themarine vibrator is towed at depth, especially in marine vibrators wherethe interior volume of the marine vibrator may be pressure balanced withthe external hydrostatic pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

These drawings illustrate certain aspects of some of the embodiments ofthe present invention and should not be used to limit or define theinvention.

FIG. 1 illustrates an example embodiment of a marine vibrator with acontainer and variable mass load.

FIG. 2 illustrates the change in the air spring effect as the pressureand volume of the internal gas is altered in accordance with exampleembodiments

FIG. 3 illustrates the shift in resonance frequency due to the airspring effect as the marine vibrator is being towed deeper in accordancewith example embodiments.

FIG. 4 illustrates a partial cross-sectional view of the marine vibratorof FIG. 1.

FIG. 5 illustrates a cross-sectional view of the marine vibrator ofFIGS. 1 and 4 taken along line 1-1 of FIG. 4.

FIG. 6 illustrates a cross-sectional view of the marine vibrator ofFIGS. 1 and 4 taken along line 2-2 of FIG. 4.

FIG. 7 illustrates a cross-sectional view of an embodiment of a marinevibrator with an alternative embodiment of a mechanical spring elementtaken along line 3-3 of FIG. 4.

FIG. 8 illustrates another example embodiment of the marine vibrator ofFIGS. 1 and 4 with a variable mass load in cross-section.

FIG. 9 illustrates a simulated amplitude spectrum showing the effect onresonance frequency of adding a mass load to a marine vibrator inaccordance example embodiments.

FIG. 10 illustrates an example embodiment of a container for adding avariable mass load to a marine vibrator in accordance with exampleembodiments.

FIG. 11 illustrates an example embodiment of a container having multiplecompartments for adding a variable mass load to a marine vibrator inaccordance with example embodiments.

FIG. 12 illustrates an example attachment of a container to a marinevibrator in accordance with example embodiments.

FIG. 13 illustrates an example embodiment of a marine seismic surveysystem using a marine vibrator.

DETAILED DESCRIPTION

It is to be understood that the present disclosure is not limited toparticular devices or methods, which may, of course, vary. It is also tobe understood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting. All numbers and ranges disclosed herein may vary by someamount. Whenever a numerical range with a lower limit and an upper limitis disclosed, any number and any included range falling within the rangeare specifically disclosed. Although individual embodiments arediscussed, the invention covers all combinations of all thoseembodiments. As used herein, the singular forms “a”, “an”, and “the”include singular and plural referents unless the content clearlydictates otherwise. Furthermore, the word “may” is used throughout thisapplication in a permissive sense (i.e., having the potential to, beingable to), not in a mandatory sense (i.e., must). The term “include,” andderivations thereof, mean “including, but not limited to.” The term“coupled” means directly or indirectly connected. If there is anyconflict in the usages of a word or term in this specification and oneor more patent or other documents that may be incorporated herein byreference, the definitions that are consistent with this specificationshould be adopted for the purposes of understanding this invention.

Embodiments relate generally to marine vibrators for marine geophysicalsurveys that incorporate one or more piston plates that may act on thesurrounding water to produce acoustic energy. In some embodiments, themarine vibrators may further comprise one or more drivers coupled to thepiston plates to cause the piston plates to move back and forth. Themarine vibrators may also include one or more springs coupled to thepiston plates and a fixture. In one or more embodiments, a variable massload may be added to a container attached to a piston plate of a marinevibrator. The variable mass load may be added to compensate for airspring effects. As discussed in more detail below, the variable massload may shift the resonance frequency of the marine vibrator lower toalleviate problems due to pressure increases in the marine vibrator.Advantageously, the marine vibrators may display a low resonancefrequency in the seismic frequency range of interest. In particularembodiments, the marine vibrators may display a first resonancefrequency (when submerged in water at a depth of from about 0 meters toabout 300 meters) within the seismic frequency range of about 1 Hz toabout 10 Hz.

Piston-type marine vibrators, which may include an actuator and aspring, act as mechanical transformers, which transform the displacementand force generated in the active element to meet the demands ofdifferent applications. Piston-type marine vibrators are generallymarine vibrators having a piston plate that vibrates to generateacoustic energy.

FIG. 1 is an example embodiment of a piston-type marine vibrator,illustrated as marine vibrator 5. As illustrated, marine vibrator 5 maycomprise piston plate 10 and container 15. In the illustratedembodiment, the container 15 may be configured to contain a variablemass load 20. In embodiments, marine vibrator 5 comprises an internalgas pressure. By way of example, marine vibrator 5 may define aninternal volume in which a gas may be disposed, this internal volume ofgas providing the internal gas pressure of marine vibrator 5. In someembodiments, marine vibrator 5 may have a pressure compensation system.The pressure compensation system may be used, for example, to equalizethe internal gas pressure of marine vibrator 5 with the externalpressure. The internal gas pressure of marine vibrator 5 will bereferred to herein as the “marine vibrator internal gas pressure.”Pressure compensation may be used, for example, where marine vibrator 5needs to be towed at depth to achieve a given level of output. As thedepth of marine vibrator 5 increases, the internal gas pressure may beincreased to equalize pressure with the increasing external pressure. Agas (e.g., air) may be introduced into marine vibrator 5, for example,to increase the internal gas pressure.

Without being limited by theory, increasing the marine vibrator internalgas pressure may create an air-spring effect that undesirably impactsthe resonance frequency of marine vibrator 5. In particular, theresonance frequency may increase as the marine vibrator internal gaspressure increases. Those of ordinary skill in the art, with the benefitof this disclosure, should appreciate that an increase in the marinevibrator internal gas pressure may also result in an increase of thebulk modulus or air-spring effect of the gas (e.g., air) in the marinevibrator 5. Among other things, the resonance frequency of marinevibrator 5 is based on the combination of the air spring of the gas inmarine vibrator 5 and the mechanical spring element (e.g., mechanicalspring elements 65 on FIG. 4) in the marine vibrator 5. Thus, increasingthe bulk modulus or air-spring effect of the internal gas of marinevibrator 5 may also result in an increase in the resonance frequency. Assuch, the resonance frequency of a marine vibrator 5 towed at depth mayundesirably increase when the marine vibrator internal gas pressure iscompensated by equalization with the external pressure (e.g., by using apressure compensation system).

FIGS. 2 and 3 illustrate the effect of an air spring on marine vibrator5 at various depths in accordance with example embodiments. In FIG. 2,the volume of the internal gas of marine vibrator 5 is represented byreference number 25. To illustrate the air spring effect, volume 25 ofthe internal gas is shown at ambient pressure at 30, under compressionat 35, and under expansion at 40. FIG. 2 therefore illustrates therelationship between pressure and volume in relation to the air springeffect. Thus, and assuming a constant temperature, as volume 25 ofincreases, the pressure of the internal gas will decrease as will theair spring effect. Conversely, as volume 25 decreases, the pressure ofthe internal gas will increase and so too will the air spring effect.With respect to FIG. 3, the curve shown at 45 is a hypotheticalrepresentation of the acoustic energy output of marine vibrator 5 at Dmeters depth without pressure compensation. The curve shown at 50represents the output of marine vibrator 5 at D+x meters depth withpressure compensation. Pressure compensation may cause an increase inthe internal gas pressure, and thus a resulting increase in the airspring effect. As illustrated by FIG. 3, the resonance frequency ofmarine vibrator 5 may shift higher with pressure compensation, thusdemonstrating how an increase in the air spring effect may result in ahigher resonance frequency. As illustrated, the increase in resonancefrequency becomes more pronounced at greater depths.

To compensate for these changes in the internal gas pressure, variablemass load 20 may be a component of marine vibrator 5. By way of example,variable mass load 20 may be added to piston plate 10 of marine vibrator5 to shift the resonance frequency lower. In some embodiments, variablemass load 20 may increase in mass with increasing depth of marinevibrator 5 in water. In particular embodiments, variable mass load 20may be implemented into marine vibrator 5 via container 15 attached topiston plate 10 of marine vibrator 5. Container 15 may be configured tofill with water as marine vibrator 5 is lowered into the water. Inembodiments, variable mass load 20 may be added outside of piston plate10. Variable mass load 20 may be appropriately sized to compensate forthe entire frequency change due to increasing depth, resulting in thesame resonance frequency independent of water depth.

Turning now to FIGS. 4-6, and with additional reference to FIG. 1, anembodiment of marine vibrator 5 is described. FIG. 4 is a partialcross-sectional view of the embodiment of marine vibrator 5 of FIG. 1with container 15, variable mass load 20, and one of the piston plates10 on one side of the marine vibrator 5 removed for the ease ofdescription.

FIG. 5 is a cross-sectional view of the embodiment of marine vibrator 5of FIGS. 1 and 4 taken along line 1-1 of FIG. 4. FIG. 6 is across-sectional view of the embodiment of marine vibrator 5 of FIGS. 1,4, and 5 taken along line 2-2 of FIG. 4.

In the illustrated embodiment, marine vibrator 5 includes a containmenthousing 55. Piston plates 10 may be flexibly coupled to containmenthousing 55, for example, by way of rubber seals 60. As best seen inFIGS. 4-6, piston plates 10 may each have mechanical spring elements 65attached to them. One or more drivers 70 may be disposed in containmenthousing 55 to cause the piston plates 10 to move back and forth. Thismotion of piston plates 10 may take advantage of the flexibility ofrubber seals 60. As would be understood by one of ordinary skill in theart with the benefit of this disclosure, rubber seals 60 do not need tobe made of rubber, but rather may be made from any material that allowsa flexible coupling of piston plates 10 to containment housing 55 asfurther discussed below.

Containment housing 55 may have first surface 75 and second surface 80,which may be opposing one another. As best seen on FIGS. 4-6, firstopening 85 and second opening 90 may be formed respectively in the firstsurface 75 and the second surface 80. While not illustrated, embodimentsmay include windows or openings 85, 90 that may be larger or smallerthan piston plates 10. Marine vibrator 5 further comprises an interiorvolume 95 which may be at least partially defined by containment housing55 and piston plates 10. In some embodiments, mechanical spring elements65 and drivers 70 may be at least partially disposed within interiorvolume 95. In alternative embodiments, mechanical spring elements 65 anddrivers 70 may be entirely disposed within interior volume 95. While notillustrated, in further alternative embodiments, mechanical springelements 65 may be disposed outside containment housing 55 so long asmechanical spring elements 65 are coupled to fixture 125. In someembodiments, marine vibrator 5 may be pressure compensated such that thepressure within interior volume 95 may be kept the same as the externalpressure (i.e. the pressure on the side of piston plate 10 opposite thatof interior volume 95), thus enabling operation at greater depth, forexample, up to about 300 meters or more. Containment housing 55 togetherwith piston plates 10 and rubber seals 60 may form a waterproof housingfor the other components of marine vibrator 5, such as mechanical springelements 65 and drivers 70. Containment housing 55 may be constructedfrom any suitable material, including, without limitation, steel (e.g.,stainless steel), aluminum, a copper alloy, glass-fiber reinforcedplastic (e.g., glass-fiber reinforced epoxy), carbon fiber reinforcedplastic, and combinations thereof. Similarly, containment housing 55 asbest seen in FIGS. 1 and 4-6, may have the general shape of arectangular box. It should be understood that other configurations ofcontainment housing 55 may be suitable, including those having thegeneral shape of a square box or other suitable shapes.

As best seen in FIG. 1, in some embodiments, containment housing 55 mayfurther include optional caps 56, which may be disposed in a lateralside of containment housing 55. In particular embodiments, one or moreof caps 56 may be removable. By way of example, caps 56 may facilitateattachment of a device, such as a compliance chamber, to containmenthousing 55. As further illustrated by FIG. 1, containment housing 55 mayinclude first and second ends 57, 58 to which brackets 59 may beseparately mounted. Brackets 59 may be used for hoisting marine vibrator5, for example when deploying marine vibrator 5 in the water. By way ofexample, brackets 59 may facilitate attachment of marine vibrator 5 totow lines, a survey vessel (e.g., survey vessel 225 on FIG. 13), orother suitable device or mechanism used in conjunction with towingmarine vibrator 5 through a body of water.

Piston plates 10 may typically be constructed of a material that willnot deform, bend or flex when in use. By way of example, piston plates10 may comprise, without limitation, steel (e.g., stainless steel),aluminum, a copper alloy, glass-fiber reinforced plastic (e.g.,glass-fiber reinforced epoxy), carbon fiber reinforced plastic, andcombinations thereof In some embodiments, piston plates 10 may besubstantially flat and rectangular in shape. By way of example, pistonplate 10 shown on FIG. 1 is rectangular in shape except with roundedcorners. In some embodiments, piston plates 10 may in the form of flat,circular disks. By way of example, piston plates 10 may each be a flat,circular disk having substantially uniform thickness. However, otherconfigurations, including both axially-symmetric and not, of pistonplates 10 may be suitable for particular applications. By way ofexample, piston plates 10 may be square, elliptical, or other suitableshape for providing the desired acoustic energy. In alternativeembodiments, piston plates 10 may be curved, either convexly protrudinginto interior volume 95, or concavely expanding interior volume 95. Ingeneral, piston plates 10 have a thickness that provides stiffness andalso withstands expected pressures. As will be appreciated by those ofordinary skill in the art with the benefit of this disclosure, the platethickness may vary based on the material of construction, among otherfactors. As will be discussed in more detail below, the mass load ofpiston plates 10 and the spring constant of mechanical spring elements65 may be selected (i.e. tuned) in a manner to produce a first resonancefrequency within the desired seismic frequency range when marinevibrator 5 is submerged in water at a depth of from about 0 meters toabout 300 meters. While a single piston plate 10 is illustrated oneither side of fixture 125, embodiments may include more than one pistonplate 10 on either side of fixture 125. Moreover, embodiments mayinclude piston plates 10 that are smaller in size with respect tocontainment housing 55 as compared to those illustrated on FIGS. 1 and4-6.

With continued reference to FIGS. 1 and 4-6, piston plates 10 may eachbe secured to containment housing 55 in a manner that allows movement ofpiston plates 10 relative to containment housing 55 with substantiallyno bending or flexing of piston plates 10. In the embodiment of FIG. 1,a pair of piston plates 10 is shown. One of the piston plates 10 may bedisposed on one side of containment housing 55 while the other pistonplates 10 may be disposed on the opposing side of containment housing55. As illustrated, one of the piston plates 10 may be coupled to thecontainment housing 55 at or near the first surface 75 and the otherpiston plate 10 may be coupled to the containment housing 55 at or nearthe second surface 80. Piston plates 10 may each cover a correspondingone of the first opening 85 or second opening 90 in the respective firstsurface 75 and second surface 80 of containment housing 55. In theillustrated embodiment, piston plates 10 are coupled to containmenthousing 55 by way of rubber seals 60. Rubber seals 60 may not holdpiston plates 10 in place but rather may flex (or otherwise move) topermit movement of piston plates 10 at their outer edges. In particularembodiments, piston plates 10 may function as piston transducers,wherein each of the piston plates 10 moves back forth by actuation ofthe drivers 70. Movement of pistons plates 10 is illustrated in FIGS. 5and 6 by arrows 100. In contrast to flextensional-shell type marinevibrators, piston plates 10 may not bend or flex in operation, butrather may move back and forth acting against the surrounding water.

Turning again to FIGS. 1 and 4-6, drivers 70, may be one of a variety oftypes of drivers 70, for example electro-dynamic drivers. In someembodiments, the drivers 70 may be “moving coil” or “voice coil”drivers, which may provide the ability to generate very large acousticenergy amplitudes. Although the particular embodiment described hereinshows four uni-directional drivers utilized in parallel, embodiments inwhich one or more bi-directional drivers, embodiments with one or moreuni-directional drivers, or embodiments in which more or less than fouruni-directional drivers are utilized, are each within the scope of theinvention. As best seen in FIGS. 5 and 6, a pair of drivers 70 may becoupled to an interior surface 105 of one piston plate 10, while anotherpair of drivers 70 may be coupled to an interior surface 105 of theother piston plate 10. Drivers 70 may also be coupled to fixture 125.

As illustrated, drivers 70 may each comprise a uni-directional, movingcoil driver, comprising an electric coil 110, transmission element 115,and magnetic circuitry 120, which work together to generate a magneticfield. As illustrated, magnetic circuitry 120 may be connected tofixture 125, while transmission element 115 may connect to thecorresponding piston plate 10. In some embodiments (not illustrated),this arrangement may be reversed (i.e., magnetic circuitry 120 connectsto the corresponding piston plate 10, while transmission element 115connects to fixture 125). As illustrated, each transmission element 115may transfer the motion of the corresponding electric coil 110 tointerior surface 105 of the corresponding piston plate 10. Whenelectrical current I is applied to electric coil 110, a force F actingon electric coil 110 may be generated as follows:

F=IlB   (Eq. 1)

Where I is the current, l is the length of the conductor in electriccoil 110, and B is the magnetic flux generated by magnetic circuitry120. By varying the magnitude of the electrical current and consequentlythe magnitude of the force acting on electric coil 110, the length ofthe driver stroke may vary. Each driver 70 may provide stroke lengths ofseveral inches up to and including about 10″ which may allow the marinevibrator 5 to generate enhanced amplitude acoustic energy output in thelow frequency ranges, for example, between about 1 Hz and about 10 Hzwhen marine vibrator 5 is submerged in water at a depth of from about 0meters to about 300 meters. Magnetic circuitry 120 may comprisepermanent magnets, though any device capable of generating a magneticflux may be incorporated.

In the illustrated embodiment, mechanical spring elements 65 (e.g., inthe form of coil springs) are disposed in containment housing 55 oneither side of fixture 125. As best seen in FIG. 6, pairs of mechanicalspring elements 65 may be located in either side of fixture 125, with afirst pair of mechanical spring elements 65 disposed on one side offixture 125, and a second pair of mechanical spring elements 65 may bedisposed on the opposing side of fixture 125. Mechanical spring elements65 in the first pair may be disposed on opposite sides of the drivers 70from one another, and mechanical spring elements 65 in the second pairmay also be disposed on opposite sides of the drivers 70 from oneanother. Mechanical spring elements 65 may each extend between acorresponding one of piston plates 10 and fixture 125. Mechanical springelements 65 may be coupled to fixture 125 and at least one of pistonplates 10 to exert a biasing action against piston plates 10. A widevariety of different mechanical spring elements 65 may be used that aresuitable for exerting the desired biasing action against piston plates10, including both linear and non-linear springs. In particularembodiments, mechanical spring elements 65 may be any of a variety ofdifferent types of springs, including compression springs, torsionsprings, or other suitable springs for exerting the desired biasingaction. Specific examples of mechanical spring elements 65 that may beused include coil springs, flat springs, bow springs, and leaf springs,among others. Suitable mechanical spring elements 65 may be constructedfrom spring steel or other suitable resilient material, such asglass-fiber reinforced plastic (e.g., glass-fiber reinforced epoxy),carbon fiber reinforced plastic, and combinations thereof. In someembodiments, the dimensions, material make-up, and the shape ofmechanical spring elements 65 may be selected to provide a sufficientspring constant for vibrations in the seismic frequency range ofinterest when the marine vibrator 5 is submerged in water at a depth offrom about 0 meters to about 300 meters.

In the illustrated embodiment, marine vibrator 5 may further includevariable mass load 20 implemented by container 15 attached to pistonplates 10. Container 15 may be attached to an exterior surface 130 ofpiston plates 10. While not shown, container 15 may include holes orother openings formed therein. These holes may be fitted with valvesthat may be operated remotely to adjust the amount of water, and thusvary the mass load, that will be allowed in to the differentcompartments of container 15. By using this method, the resonancefrequency may be adjusted depending on the depth. Accordingly, whenmarine vibrator 5 is lowered into water, water may enter container 15,and thus increase the variable mass load 20 of container 15. In thismanner, variable mass load 20 may be variable based on the amount ofwater in container 15. Therefore, the resonance frequency for marinevibrator 5 may be selected based at least in part on variable mass load20.

In some embodiments, a fixture 125 suspends drivers 70 withincontainment housing 55. For example, in the illustrated embodiment,fixture 125 extends along the major axis of containment housing 55 andmay be coupled to either end of containment housing 55. Fixture 125 maybe circular, square, rectangular, or other suitable cross-section asdesired for a particular application. An example of a suitable fixture125 may include a rod, beam, plate, or other suitable frame forsupporting internal components such as drivers 70 in containment housing55. In particular embodiments, fixture 125 should be fixed tocontainment housing 55 in a mariner that restricts movement andtherefore prevents undesired contraction of the major axis ofcontainment housing 55. In particular embodiments, piston plates 10 maywork in symmetry above and below fixture 125. In other words, in someembodiments, fixture 125 may divide marine vibrator 5 into symmetricalhalves with respect to at least the piston plates 10, mechanical springelements 65, and drivers 70.

In the illustrated embodiment, coupling of rubber seals 60 to pistonplates 10 is shown. Rubber seals 60 may also be coupled to containmenthousing 55, for example, to form a water-tight seal between pistonplates 10 and containment housing 55. In general, rubber seals 60 may beconfigured to allow movement of piston plates 10 while also maintainingthe appropriate seal. Rubber seals 60 may have significant curvature topermit significant amplitude of movement. By way of example, thispermitted movement may further enable piston plates 10 to have severalinches of travel, e.g., piston plates 10 may move back and forthrelative to containment housing 55 a distance of from about 1 inch toabout 10 inches (or more). Other techniques for permitting movement maybe used, including the use of seals with bellows or accordion-typeconfigurations.

As would be understood by one of ordinary skill in the art, the totalimpedance that may be experienced by a marine vibrator 5 may beexpressed as follows:

Z _(r) =R _(r) +jX _(r)   (Eq. 2)

where Z_(r) is total impedance, R_(r) is radiation impedance, and X_(r)is reactive impedance.

In an analysis of the energy transfer of the marine vibrator 5, thesystem may be approximated as a baffled piston. In the expression of thetotal impedance that will be experienced, the radiation impedance R_(r)of a baffled piston may be:

R _(r) =πa ²ρ_(o) cR ₁(x)   (Eq. 3)

and the reactive impedance may be:

X _(r) =πa ²ρ_(o) cX ₁(x)   (Eq. 4)

where

x=2ka=(4πa/λ)=(2Ωa/c)   (Eq. 5)

and where

$\begin{matrix}{{R_{1}(x)} = {1 - {\left( {2/x} \right)J_{1{(x)}}\mspace{14mu} {and}}}} & \left( {{Eq}.\mspace{14mu} 6} \right) \\{{X_{1}(x)} = {\left( \frac{4}{\pi} \right){\int_{0}^{\pi/2}{{\sin \left( {x\mspace{14mu} \cos \mspace{14mu} \alpha} \right)}\sin^{2}\alpha \ {\alpha}}}}} & \left( {{Eq}.\mspace{14mu} 7} \right)\end{matrix}$

where ρ_(o) is the density of water, Ω=radial frequency, k=wave number,a=radius of piston, c=sound velocity, λ=wave length, and J₁=Besselfunction of the first order.

Using the Taylor series expansion on the above equations yields thefollowing:

$\begin{matrix}{{R_{1}(x)} = {\frac{x^{2}}{2^{2}{1!}{2!}} - \frac{x^{4}}{2^{4}{2!}{3!}} +}} & \left( {{Eq}.\mspace{14mu} 8} \right) \\{{X_{1}(x)} = {\frac{4}{\pi}\left( {\frac{x}{3} - \frac{x^{3}}{3^{2}5} + \frac{x^{5}}{3^{2}5^{2}7} -} \right.}} & \left( {{Eq}.\mspace{14mu} 9} \right)\end{matrix}$

For low frequencies, when x=2ka is much smaller than 1, the real andimaginary part of the total impedance expression may be approximatedwith the first term of the Taylor expression. The expressions for lowfrequencies, when the wave length is much larger than the radius of thepiston becomes:

R ₁(x)=(1/2)(ka)²   (Eq. 10)

X ₁(x)→(8ka)/(3π)   (Eq. 11)

It follows that, for low frequencies, R will be a small number comparedto X, which suggests a very low efficiency signal generation. However,embodiments may introduce a resonance in the lower end of the frequencyspectrum so that low frequency acoustic energy may be generated moreefficiently. At resonance, the imaginary (reactive) part of theimpedance is cancelled, and the marine vibrator may be able toefficiently transmit acoustic energy into the body of water.

FIG. 7 illustrates a cross-sectional view of one embodiment of marinevibrator 5 that comprises an alternative embodiment of mechanical springelement 65. This cross-sectional view is taken along line 3-3 of FIG. 4.In contrast to the mechanical spring elements of FIGS. 4-6 which areillustrated as coiled springs, FIG. 7 illustrates mechanical springelements 65 in the form of a bow spring. In this cross-sectional view ofFIG. 7, certain elements of marine vibrator 5, such as the drivers 70,are not visible.

The following description is for one of mechanical spring elements 65;however, because fixture 125 provides a line of symmetry, thisdescription is equally applicable to both of mechanical spring elements65. As illustrated in FIG. 7, one of mechanical spring elements 65 maybe coupled to one of piston plates 10 and fixture 125. Mechanical springelement 65 may be coupled to piston plate 10 at attachment point 135,which may be a fixed connection, for example, that does not permitmovement. Mechanical spring element 65 may be coupled to supplementalfixture 140, which may be in the form of a beam, rod, or other suitableframe for supporting mechanical spring element 65 in containment housing55. Mechanical spring element 65 may be coupled to supplemental fixture140 by way of bearings 145. In particular embodiments, bearings 145 maybe linear bearings that permit linear movement of the ends of mechanicalspring element 65 as represented by arrows 150. In this manner,mechanical spring element 65 may be allowed to flex and provide abiasing force to piston plate 10 upon its movement. Supplemental fixture140 may be coupled to fixture 125 at one or more of fixture attachmentpoints 155, which may be fixed connections that do not permit movement.Additionally, marine vibrator 5 of FIG. 7 is illustrated with a variablemass load 20 implemented via container 15 attached to exterior surface130 of piston plate 10 in a substantially similar manner as wasillustrated in FIGS. 1, 5, and 6. As in FIGS. 1, 5, and 6, variable massload 20 may be variable based on the amount of water in container 15.Therefore, the resonance frequency for marine vibrator 5 is selectedbased at least in part on the variable mass load 20.

Turning now to FIG. 8, marine vibrator 5 is illustrated as furthercomprising two mass spring elements 160 with weights 165 affixedthereto. Mass springs elements 160 shown on FIG. 8 may also be used inconjunction with the mechanical spring elements 65 shown on FIG. 7 (orother suitable type of mechanical spring element 65). As illustrated,mass spring elements 160 may be generally elliptically shaped. Asillustrated, mass spring elements 160 may be coupled to fixture 125 andpiston plates 10. In the illustrated embodiment, a pair of mass springelements 160 are shown on either side of fixture 125 so that marinevibrator 5 comprises four mass spring elements 160. However, it shouldbe understood that more or less than four mass spring elements 160 maybe utilized for a particular application. As will be described below, invarious embodiments, the spring constant of mass spring elements 160 andthe mass of weights 165 may be selected in a manner to achieve a secondsystem resonance frequency within the seismic frequency range ofinterest when marine vibrator 5 is submerged in water at a depth of fromabout 0 meters to about 300 meters. In a particular embodiment, marinevibrator 5 may exhibit a first resonance frequency of about 2.5 Hz and asecond resonance frequency of about 4.5 Hz when submerged in water at adepth of from about 0 meters to about 300 meters. Although a marinevibrator 5 that does not include mass spring elements 160, as shown inthe embodiment illustrated in FIGS. 4-6, may display a second resonancefrequency, the second resonance frequency would typically be much higherand thus outside the seismic frequency range of interest. Additionally,marine vibrator 5 of FIG. 8 is illustrated with a variable mass load 20implemented via container 15 attached to exterior surface 130 of pistonplate 10 in a substantially similar manner as was illustrated in FIGS.1, 5, and 6. As in FIGS. 1, 5, and 6, variable mass load 20 may bevariable based on the amount of water in container 15. Therefore, theresonance frequency for marine vibrator 5 is selected based at least inpart on the variable mass load 20.

In some embodiments, the marine vibrator 5 may display at least oneresonance frequency (when submerged in water at a depth of from about 0meters to about 300 meters) between about 1 Hz to about 200 Hz. Inalternative embodiments, the marine vibrator 5 may display at least oneresonance frequency (when submerged in water at a depth of from about 0meters to about 300 meters) between about 0.1 Hz and about 100 Hz,alternatively, between about 0.1 Hz and about 10 Hz, and alternatively,between about 0.1 Hz and about 5 Hz. In some embodiment, the marinevibrator 5 may display at least two resonance frequencies of about 10 Hzor lower. The first resonance frequency may result substantially frominteraction of the outer piston plate 10 and the mechanical springelement 65. The second resonance frequency may result substantially fromthe interaction of the mass spring elements 160 with the added weights165.

FIG. 9 illustrates the results from a simulation of the first resonancefrequency of marine vibrator 5 towed at 50 meters. FIG. 9 represents theoutput of marine vibrator 5 comprising a variable mass load 20 of 0kilograms (“kg”), 1000 kg, 1500 kg, and 2,000 kg respectively. Asillustrated, the addition of variable mass load 20 decreased theresonance frequency, more particularly; the resonance of marine vibrator5 was shifted from 3.4 Hz to 2.7 Hz with the addition of 2000 kg. Below2 Hz, there was very little difference in the sound output.

In evaluating the addition of a variable mass load 20, finite elementanalysis may be utilized as known to those of ordinary skill in the art.In such an analysis, the following principles may be relevant. If pistonplate 10 of marine vibrator 5 is approximated as a baffled piston, then,for low frequencies, variable mass load 20, or the equivalent fluid massacting on piston plate 10 may be:

M _(piston)=ρ_(o)(8a ³/3)  (Eq. 12)

where M_(piston) is the mass load acting on piston plate 10, ρ_(o) isthe density of water surrounding marine vibrator 5, and a is theequivalent radius for a piston plate which corresponds to the size ofpiston plate 10.

Mechanical spring elements 65 may also have a spring constant in thedirection of the moving electric coils (e.g., electric coil 110 on FIGS.4-6). Therefore, with the addition of variable mass load 20, the firstresonance f_(resonance-1), due to the interaction of piston plate 10 andits mechanical spring element 65 may be substantially determined by thefollowing mass spring relationship:

$\begin{matrix}{f_{{resonance} - 1} = {\frac{1}{2\pi}\sqrt{\frac{K_{{piston}\_ {spring}}}{M_{piston} + M_{{mass}\mspace{14mu} {load}}}}}} & \left( {{Eq}.\mspace{14mu} 13} \right)\end{matrix}$

where K_(piston) _(_) _(spring) is the spring constant of mechanicalspring element 65 attached to piston plate 10, M_(piston) is the massload of piston plate 10, and M_(mass load) is variable mass load 20.

To achieve efficient energy transmission in the seismic frequency rangeof interest, it may be desirable to achieve a second resonance frequencywithin the seismic frequency range of interest. In the absence of massspring elements 160 (as shown in FIG. 8) with added weights 165 (also asshown in FIG. 8), the second resonance frequency would occur when pistonplate 10 has its second Eigen-mode. This resonance frequency, however,is normally much higher than the first resonance frequency and notdesirable, and accordingly, would typically be outside the seismicfrequency range of interest. As is evident from the foregoing equation,the resonance frequency will be reduced if variable mass load 20 isincreased. However, in order to add sufficient mass to achieve a secondresonance frequency within the seismic frequency range of interest, theamount of mass required in variable mass load 20 to achieve a desirablesecond resonance frequency may make such a system less practical for usein marine seismic surveying operations.

Therefore, in some embodiments, mass spring elements 160 may be includedinside marine vibrator 5 with added weights 165 on the side of the massspring elements 160. Mass spring elements 160 may have a transformationfactor T_(spring) between the long and short axis of its ellipse, sothat the deflection of the two side portions will have a higheramplitude than the deflection of the end attached to piston plate 10 anddriver 70.

The effect of such added weights 165 is equivalent to adding mass on theend of driver 70 where it is attached to piston plate 10.

M _(spring)=(T _(spring))² ·M _(added)   (Eq. 14)

Use of mass spring elements 160 with added weights 165, may allow thesecond resonance frequency of the system to be tuned so that the secondresonance frequency is within the seismic frequency range of interest,thereby improving the efficiency of the marine vibrator 5 in the seismicfrequency range of interest.

$\begin{matrix}{f_{{resonance}\; 2} = {\frac{1}{2\pi}\sqrt{\frac{K_{spring} + K_{{piston}\_ {spring}}}{{\left( T_{spring} \right)^{2} \cdot M_{added} \cdot {+ M_{shell}}} + M_{{mass}\mspace{14mu} {load}}}}}} & \left( {{Eq}.\mspace{14mu} 15} \right)\end{matrix}$

where K_(spring) is the spring constant of mass spring elements 160, andK_(piston) _(_) _(spring) is the spring constant of the mechanicalspring element 65 attached to piston plate 10.

Accordingly, it may be possible, as shown above, to select weights 165on mass spring elements 160 to tune the second resonance frequency. Itmay also be possible to select the extent of influence the secondresonance frequency may have on the system. By way of example, if massspring elements 160 have low spring constants compared to mechanicalspring element 65 attached to piston plate 10, and a matching weight 165is added to mass spring elements 160, mass spring elements 160 withweights 165 will function relatively independently from mechanicalspring element 65 attached piston plate 10. In such cases, the secondresonance frequency may be as follows:

$\begin{matrix}{f_{{resonance}\; 2} = {\frac{1}{2\pi}\sqrt{\frac{K_{spring}}{\left( T_{spring} \right)^{2} \cdot M_{added}}}}} & \left( {{Eq}.\mspace{14mu} 16} \right)\end{matrix}$

In the same way, it may also be possible in some embodiments to make thesecond resonance frequency very large by selecting a high springconstant for mass spring elements 160 with a matching weight 165 suchthat the second resonance frequency will have a larger amplitude thanthe first resonance frequency.

FIG. 10 illustrates container 15 for adding variable mass load 20 tomarine vibrator 5 (e.g., as illustrated on FIGS. 1 and 5-8) inaccordance with some embodiments. An interior volume may be formedinside container 15, for example, to hold water. As illustrated,container 15 may have at least one water inlet 170 to allow ingress ofwater into the interior volume of container 15. When container 15 issubmerged in water, water may enter the container 15 by way of the waterinlet 170 to add variable mass load 20 to marine vibrator 5. The watermay exit container 15 as it is lifted out of the water. In this manner,container 15 may add little mass to marine vibrator 5 at the surface butmay add mass to marine vibrator 5 at depth based on the amount of waterin container 15. It should be understood that while only a single waterinlet 170 is shown on FIG. 10, the particular configuration and numberof water inlets 170 in container 15 may be varied as desired for aparticular application. The number and configuration of water inlets 170may be selected, for example, based on the desired rate and amount ofwater ingress into (or egress from) container 15. In some embodiments,container 15 may be sized to hold from about 0.1 m³ to about 4 m³ ofwater in volume and about 1 m³ of water in some embodiments.

FIG. 11 illustrates a cross-sectional view of container 15 in accordancewith certain embodiments. As illustrated, the interior volume ofcontainer 15 may be divided into a plurality of compartments 175 a to175 d. Compartments 175 a to 175 d may be separated by internal walls180. Valves 185 may be located in internal walls 180 which may be openedor closed to allow compartments 175 a to 175 d to selectively fill atdifferent depths under water. In some embodiments, valves 185 may beremotely operated. Accordingly, container 15 may be optimized to providea variable mass load 20 to marine vibrator 5 based on depth, thusallowing optimization of the resonance frequency at greater depths. Forexample, if marine vibrator 5 is towed at 40 meters, only a portion ofcontainer 15 may be filled with water, whereas if marine vibrator 5 istowed at 120 meters, the entirety of container 15 may be filled withwater.

FIG. 12 illustrates attachment of container 15 to marine vibrator 5 inaccordance with certain embodiments. For simplicity, marine vibrator 5is shown without an inner structure, such as a driver (e.g., driver 70shown on FIG. 4) and other internal components. As illustrated,container 15 may be attached to outer piston plate 10 of marine vibrator5. It may desirable for container 15 to be attached as close as possibleto marine vibrator 5 without contact (other than at attachment points),which may undesirably impact performance of marine vibrator 5. In someembodiments, lower surface 190 of container 15 may be a distance ofabout 1 centimeter or less from piston plate 10. It is to be understoodthat embodiments may include attachment of multiple containers 15 to anyof the piston plates 10. As illustrated, container 15 may be attached tothe marine vibrator 5 using one or more pin joints 195. In theillustrated embodiment, container 15 is attached at one or more pointsalong the midline of piston plate 10. By placement of container 15 atthe midline of piston plate 10, the resonance frequency of marinevibrator 5 may be shifted in accordance with example embodiments.Container 15 is considered to be attached at substantially the midlineif the attachment is a distance from the middle of no more than 20% ofthe width of piston plate 10 between an edge of piston plate 10.

In the illustrated embodiment, container 15 is further coupled to frame200. Coupling of container 15 to frame 200 may prevent tilting ofcontainer 15, for example, as marine vibrator 5 may be towed throughwater. As illustrated, frame 200 may surround marine vibrator 5.Container 15 may be coupled to frame 200 via one or more connectingsprings 205. In some embodiments, connecting springs 205 may attach totab elements 210 of container 15. For example, connecting springs 205may include hooked portions 215 that are secured in holes 220 of tabelements 210.

FIG. 13 illustrates an example technique for acquiring marine seismicdata that may be used with embodiments of the present techniques. In theillustrated embodiment, a survey vessel 225 moves along the surface of abody of water 230, such as a lake or ocean. The survey vessel 225 mayinclude thereon equipment, shown generally at 235 and collectivelyreferred to herein as a “recording system.” The recording system 235 mayinclude devices (none shown separately) for detecting and making a timeindexed record of signals generated by each of seismic sensors 240(explained further below) and for actuating a marine vibrator 5 atselected times. The recording system 235 may also include devices (noneshown separately) for determining the geodetic position of the surveyvessel 225 and the various seismic sensors 240.

As illustrated, survey vessel 225 (or a different vessel) may tow marinevibrator 5 in body of water 230. Source cable 245 may couple marinevibrator 5 to survey vessel 225. Marine vibrator 5 may be towed in bodyof water 230 at a depth ranging from 0 meters to about 300 meters, forexample. While only a single marine vibrator 5 is shown in FIG. 13, itis contemplated that embodiments may include more than one marinevibrator 5 (or other type of sound source) towed by survey vessel 225 ora different vessel. In some embodiments, one or more arrays of marinevibrators 5 may be used. At selected times, marine vibrator 5 may betriggered, for example, by recording system 235, to generate acousticenergy. Survey vessel 225 (or a different vessel) may further tow atleast one sensor streamer 250 to detect the acoustic energy thatoriginated from marine vibrator 5 after it has interacted, for example,with rock formations 255 below water bottom 260. As illustrated, bothmarine vibrator 5 and sensor streamer 250 may be towed above waterbottom 260. Sensor streamer 250 may contain seismic sensors 240 thereonat spaced apart locations. In some embodiments, more than one sensorstreamer 250 may be towed by survey vessel 225, which may be spacedapart laterally, vertically, or both laterally and vertically. While notshown, some marine seismic surveys locate the seismic sensors 240 onocean bottom cables or nodes in addition to, or instead of, a sensorstreamer 250. Seismic sensors 240 may be any type of seismic sensorsknown in the art, including hydrophones, geophones, particle velocitysensors, particle displacement sensors, particle acceleration sensors,or pressure gradient sensors, for example. By way of example, seismicsensors 240 may generate response signals, such as electrical or opticalsignals, in response to detected acoustic energy. Signals generated byseismic sensors 240 may be communicated to recording system 235. Thedetected energy may be used to infer certain properties of thesubsurface rock, such as structure, mineral composition and fluidcontent, thereby providing information useful in the recovery ofhydrocarbons.

In accordance with an embodiment of the invention, a geophysical dataproduct may be produced. The geophysical data product may includegeophysical data that is obtained by a process that includes detectingthe acoustic energy originating from marine vibrator 5. The geophysicaldata product may be stored on a non-transitory, tangiblecomputer-readable medium. The geophysical data product may be producedoffshore (i.e. by equipment on a vessel) or onshore (i.e. at a facilityon land) either within the United States or in another country. If thegeophysical data product is produced offshore or in another country, itmay be imported onshore to a facility in the United States. Once onshorein the United States, geophysical analysis, including further dataprocessing, may be performed on the data product.

The foregoing figures and discussion are not intended to include allfeatures of the present techniques to accommodate a buyer or seller, orto describe the system, nor is such figures and discussion limiting butexemplary and in the spirit of the present techniques.

What is claimed is:
 1. A marine vibrator comprising: a containmenthousing; a piston plate; an interior volume, wherein the interior volumeis at least partially defined by the containment housing and the pistonplate; a fixture coupled to the containment housing; a mechanical springelement coupled to the piston plate and the fixture, wherein themechanical spring element is at least partially disposed within theinterior volume; a driver disposed in the marine vibrator, wherein thedriver is coupled to the piston plate and the fixture, wherein thedriver is at least partially disposed within the interior volume; and acontainer coupled to an exterior surface of the piston plate, whereinthe container is configured to hold a variable mass load.
 2. The marinevibrator of claim 1, wherein the marine vibrator has at least oneresonance frequency of about 10 Hz or lower when submerged in water at adepth of from about 0 meters to about 300 meters.
 3. The marine vibratorof claim 1, wherein the driver comprises an electro-dynamic driver. 4.The marine vibrator of claim 1, wherein the variable mass load iscomprised of water and wherein the variable mass load increases withincreasing depth of the container.
 5. The marine vibrator of claim 1,wherein the container comprises one or more holes for ingress of waterwhen the marine vibrator is placed under water.
 6. The marine vibratorof claim 5, wherein at least some of the one or more holes compriseremotely operated valves.
 7. The marine vibrator of claim 1, wherein thecontainer comprises a plurality of compartments operable to selectivelyfill at different depths under water.
 8. The marine vibrator of claim 1,wherein the mechanical spring element comprises at least one type ofspring selected from the group consisting of: a bow spring, a coilspring, a flat spring, and a leaf spring.
 9. The marine vibrator ofclaim 1, further comprising a second mechanical spring element disposedon an opposite side of the driver from the mechanical spring element.10. The marine vibrator of claim 1, wherein the piston plate is coupledto the containment housing by way of a rubber seal, the piston platecovering an opening in a first side of the containment housing.
 11. Themarine vibrator of claim 1, wherein the marine vibrator furthercomprises a mass spring having weights affixed thereto, the mass springbeing coupled to the fixture and the piston plate.
 12. The marinevibrator of claim 1, further comprising a frame surrounding thecontainer and the piston plate, wherein the container is coupled to theframe via one or more connecting springs.
 13. A system comprising: aframe; a marine vibrator comprising: a containment housing; a fixturecoupled to the containment housing; a first piston plate; a first drivercoupled to the fixture and the first piston plate, wherein the firstdriver is configured to move the first piston plate back and forth; afirst pair of mechanical spring elements coupled to the first pistonplate and the fixture, wherein the first pair of mechanical springelements are positioned on opposite sides of the first driver from oneanother; a first container coupled to an exterior surface of the firstpiston plate, wherein the first container is configured to hold avariable mass load, wherein the first container is coupled to the frameby one or more connecting springs, wherein the connecting springs areattached to tab elements of the first container; a second piston plate;a second driver coupled to the fixture and the second piston plate,wherein the second driver is configured to move the second piston plateback and forth; a second pair of mechanical spring elements coupled tothe second piston plate and the fixture, wherein the second pair ofmechanical spring elements are positioned on opposite sides of thesecond driver from one another; and a second container coupled to anexterior surface of the second piston plate, wherein the secondcontainer is configured to hold a variable mass load.
 14. The system ofclaim 13, wherein at least one of the first container or the secondcontainer comprises one or more holes for ingress of water when themarine vibrator is placed under water.
 15. The system of claim 13,wherein at least one of the first container or the second containercomprises one or more holes with remotely operated valves
 16. The systemof claim 13, wherein at least one of the first container or the secondcontainer comprises a plurality of compartments operable to selectivelyfill at different depths under water.
 17. The system of claim 13,wherein at least one of the first pair of mechanical spring elements orthe second pair of mechanical spring elements is a bow spring.
 18. Thesystem of claim 13, wherein at least one of the first piston plate orthe second piston plate is coupled to the containment housing by way ofa rubber seal.
 19. A method comprising: towing a marine vibrator in abody of water in conjunction with a marine seismic survey; triggeringthe marine vibrator to cause one or more piston plates in the marinevibrator to move back and forth wherein one or more mechanical springelements exert a biasing force against the one or more piston plates,the one or more mechanical spring elements being coupled to the one ormore piston plates and a fixture in the marine vibrator; and optimizinga resonance frequency, wherein the optimizing a resonance frequencycomprises varying a mass load on the one or more piston plates, whereinthe varying the mass load comprises selectively filling compartments ofthe marine vibrator at different depths.
 20. The method of claim 19,further comprising: obtaining geophysical data; processing thegeophysical data to generate a geophysical data product, wherein thegeophysical data product is obtained by a process that includesdetecting acoustic energy originating from the marine vibrator;recording the geophysical data product on a tangible, non-volatilecomputer-readable medium suitable for importing onshore; and performinggeophysical analysis onshore on the geophysical data product.